1. Field of the Invention
The present invention relates to an apparatus for driving an optical system used in an optical pickup in a magnetooptic disk drive unit.
2. Description of the Related Art
An apparatus for driving an optical system has an objective lens for converging a light beam for recording or reproducing information to a recording medium composed of a vertically magnetized thin film having magnetooptic characteristics. The optical system driving apparatus further has driving means for electromagnetically moving the objective lens in the direction of an optical axis thereof or in a direction perpendicular to this optical axis. Further, there is another apparatus for driving an optical system having an objective lens for converging a light beam for recording or reproducing information to a concentric or spiral track disposed in a recording medium in the shape of a disk composed of a vertically magnetized thin film having magnetooptic characteristics. This optical system driving apparatus further has a plurality of stationary magnetic circuits in addition to the above optical system including the objective lens. Each of the magnetic circuits is composed of a permanent magnet extending in parallel with a radial direction of the above recording medium in the shape of a disk, a yoke and a clearance. The optical system driving apparatus further has a coil fixed to the above optical system and at least one portion of this coil crosses a magnetic flux within the above clearance.
In such an optical system driving apparatus, a recording magnetic field or an erasing magnetic field is reduced by a leakage magnetic flux from the magnetic circuits disposed as a means for driving the optical system when information is recorded or reproduced. Accordingly, there is a fear that no information is sufficiently recorded or erased. Further, there is a fear that the leakage magnetic flux is applied to the recording medium at the reproducing time of information so that recorded information is damaged. For example, Japanese Patent Application Laying Open (KOKAI) Nos. 59-148159 and 62-285261 show techniques for solving the above problems.
In the former technique, a magnetic shielding member having a high permeability is arranged between the recording medium and a body of an optical pickup device.
In the latter technique, a correcting magnet is disposed around a lens actuator as a means for reducing the leakage magnetic flux in the vicinity of a convergent point of the light beam.
In the above former technique, the magnetic shielding member is arranged in a small space between the recording medium and the optical pickup. Therefore, there is a fear that the recording medium and the magnetic shielding member come in contact with each other. Accordingly, high accuracy in assembly is required to solve this problem.
There is another case in which the magnetic shielding member is fixed to the optical system movably disposed in a radial direction of the recording medium such that an arbitrary track of the recording medium can be accessed. In such a case, the weight of movable members is increased and this increase in weight prevents the track from being accessed at a high speed.
In the above latter technique, similar to the former technique, the weight of movable members is increased and this increase in weight prevents the track from being accessed at a high speed since the correcting magnet is disposed around the lens actuator of the optical pickup.
As mentioned above, the magnetic shielding member, the correcting magnet, etc. are newly disposed to reduce an influence of the leakage magnetic flux from the magnetic circuits as a mechanism for driving the optical system when information is recorded and reproduced. However, in such a case, the cost of the optical system driving apparatus is increased because high accuracy in assembly is required and the number of parts is increased. Further, the speed of an access operation is reduced by the increase in weight of movable portions in the optical system.